Castor 6

Castor is a sextuple star system made of three
binaries, bright visual stars Castor Aab and
Bab and dim red flare stars YY Geminorum AB,
which are much brighter in X-ray wavelengths.
(See a Digitized Sky Survey
image
of Castor
from the
Nearby
Stars Database.)

System Summary

The distance of Castor, or Alpha Geminorum, from Sol was
estimated by the
HIPPARCOS
mission to be about 51.6 light-years (ly) using a
space-based parallax (Plx=0.06327, e_Plx= +/- 0.00123).
On the other hand, the Yale Parallax Catalogue suggests
a distance of 43.7 ly. However, it has about twice the
acknowledged error based on an average of Earth-based
observations. Given the past difficulty of obtaining
good Earth-based parallaxes noted in the literature and
for the sake of simplicity, most new calculations
presented here will use the farther HIPPARCOS distance
estimate.

In 1998, Castor was confirmed to be the title member of
the Castor Moving Group of 16 stars (which includes
Vega and
Fomalhaut) whose
stellar characteristics suggest a young average age of
around 200 (+/- 100) million years
(D.
Barrado y Navascues, 1998; and
Anosova
and Orlov, 1991). Since its birth, the Castor
system has moved away from its original cluster stars and
into into the vicinity of Pollux.
Although is Pollux actually the brightest star of Constellation
Gemini
(color
photo),
the Twins,
Johannes
Bayer (1572-1625) gave the first-rank
Greek
letter designation of Alpha to
Castor
(7:34:35.9+31:53:17.8, ICRS 2000.0) around 1600. Hence, it
has been suggested that one of these stars may have changed
in luminosity during the past four centuries. Indeed,
Castor (the "Horseman" of the two warriors) is not much like
its "twin" star Pollux. (See Akira Fujii's color photo of
Castor
-- blue star at the top left of photo.)

While Pollux is a highly evolved and relatively cool
orange-red giant, single star, Castor is actually composed
of three sets of tight, spectroscopic binary stars with a
combined luminosity of 52.4 times that of Sol's. The
Castor system has as many as four bluish-white, main
sequence stars and two known, fainter red dwarf companions.
The outer pair (YY Gem) was separated from the inner
quadruple of bright stars by over 1,100 AUs (73" at a
HIPPARCOS distance estimate of 51.6), moving in an orbit
that may take some 10,000 years to complete; with a 40
percent probability of being quasi-elliptical and a 60
percent probability of being hyperbolic
(Anosova
and Orlov, 1991). According to
Robert
Burnham, Jr. (1931-93), the star's multiplicity was
probably first resolved in 1678 by
Giovanni
Domenico Cassini (1625-1712) as the first gravitationally
bound objects observed beyond the Solar System. (See an
animation of the planetary and potentially habitable zone
orbits of the Castor
AabBab system and of
the Cab system, each
with a table of basic orbital and physical characteristics.)

Castor AabBab and Cab have been
observed to flare
(larger
x-ray images).

Broadband and frequent x-ray flares and quiescent x-ray flux
in both the inner bright stars Aab and Bab pairs as well as
the outer pair of flare stars Cab, or YY Geminorum AB, have
been repeatedly observed
(Güdel
et al, 2001;
Gotthelf
et al, 1994;
Schmitt
et al, 1994; and
Pallavicini
et al, 1990). The flares are believed to evolve in
minutes to hours from magnetic instabilities in hot
outer stellar atmospheres. While this was theoretically
and observational anticipated for young red dwarfs, it was
not expected for A-type dwarfs. Hence, the recent detection
presence of flares from the Aab and Bab systems has led to
debates over the actual spectral type of their companions
and the magnetic stability of young A dwarfs since ROSAT had
unexpectedly observed X-ray emission from early A-type dwarf
stars with post-T Tauri companions. The Castor system
(especially YY Gem) was also found to be relatively bright
in radio, microwave, and some infrared wavelengths detected
by IRAS
(Freidemann
et al, 1996). Over the coming decades, the two
optically brighter binary pairs Castor Aab and Bab will
move further apart in angular distance which will make
it easier to redetermine their spectral types and masses.

Castor Aa is a main sequence dwarf bluish-white star of
spectral and luminosity type A1 V. Given its shared
spectral type with well known neighbor
Sirius A, Star Aa
probably has a similar mass around 2.15 Solar. It may
also have around 2.3 times Sol's diameter and 17 to 34 times
its visual luminosity or more (depending on whether its
unseen companion is an A-type dwarf). All the stars in this
system may be less enriched than Sol
(Güdel
et al, 2001) -- despite being more enriched than Vega --
with elements heavier than hydrogen ("metallicity") based on
its abundance of iron
(Cayrel
de Strobel et al, 1991, pp. 286 and 303). The star has
a single-lined, spectroscopic stellar companion Ab.
Useful catalogue numbers and designations for the star include:
Alp or Alf Gem A, 66 Gem A, HR 2891*, Gl 278 A, Hip 36850, HD 60179,
BD+32 1581, LTT 12038, WDS 07346+3153 A, ADS 6175 A,
and Struve 1110 A.

Stars Aab are separated from its nearest binary companions
Bab by around 107 AUs (6.805" of a semi-major axis at a
HIPPARCOS distance estimate of 51.6 ly) in an orbit that
takes 467 years to complete. The two binary pairs move
in an eccentric orbit (e= 0.343) that is inclined by
114.5° from the perspective of an observer on Earth
(Wulff
Dieter Heintz, 1988). (See an animation of
the planetary and
potentially habitable zone orbits of the Castor Aab
system, with a table of basic orbital and physical
characteristics.)

Castor Ab is a main sequence dwarf of uncertain spectral
type, probably ranging from M5 to brighter ("earlier")
type of probable luminosity type V
(Golub
et al, 1983). Its mass probably ranges from 40
to 60 percent or more of Sol's, with a diameter and
luminosity less than Sol's (if not an A type dwarf.
Based on EXOSAT observations, this object is believed to
be a relatively bright, quiescent x-ray source that emits
broadband flares and frequent x-ray flares
(Güdel
et al, 2001;
Gotthelf
et al, 1994;
Schmitt
et al, 1994; and
Pallavicini
et al, 1990). Such unexpected, X-ray emission has
been observed with early type and post-T Tauri A dwarf
companions with ROSAT. Useful catalogue numbers and
designations for the star include: Struve 1110 C and
ADS 6175 C.

Castor Ba is a bluish-white main sequence dwarf star
of spectral and luminosity type A2-5 Vm. Other than the
metallic lines in its spectra, the star appears similar
to Fomalhaut. Star
Ba has an estimated mass around 1.7 times Sol's, 1.6
times its diameter, and over 14 times its visual
luminosity. The star has a single-lined, spectroscopic
stellar companion Bb, and frequent x-ray flaring from the
binary system has been observed
(Güdel
et al, 2001).
Useful catalogue numbers and designations for the
star include: Alp or Alf Gem B, 66 Gem B, HR 2890*,
Gl 278 B, HD 60178, SAO 60198, FK5 287,
WDS 07346+3153 B, ADS 6175 B, and Struve 1110 B.

Castor Bb is a main sequence dwarf star of uncertain
spectral type, probably ranging from M2 to a brighter
type ("earlier") -- possibly with metallic lines as
well -- and luminosity type V
(Golub
et al, 1983). Its mass probably ranges from 40
to 60 percent or more of Sol's, with a diameter and
luminosity less than Sol's. Observations with the
XMM-Newton
detected flares coming from the Bab system,
possibly from this unseen companion Bb.

Castor Ca is a red main sequence dwarf star of spectral
and luminosity type M0.5 Ve. It has an estimated mass
around 62 percent of Sol's, 76 percent of its diameter,
(Schmitt
et al, 1994; from
Joy
and Sanford, 1926) and 2.6 percent of its visual
luminosity. The star has an eclipsing, spectroscopic
stellar companion Cb. Both are considered to be flare
stars with frequent emission of x-ray flares and so
have been given the variable designation YY Geminorum.
Useful catalogue numbers and designations for the star
include: Alp or Alf Gem C, 66 Gem C, YY Gem, Gl 278 C,
and BD+32 1582.

Castor Cab may both be M-type flare stars, like
young UV Ceti
(Luyten 726-8 B) shown flaring at left.
UV Ceti is an extreme example of a flare star
that can
boost its brightness by five times in less than
a minute,
then fall somewhat slower back down to normal
luminosity within two or three minutes before flaring
suddenly again after several hours.

Castor Cb is a red main sequence dwarf star of
spectral and luminosity type M0.5 Ve. It has an
estimated mass around 57 percent of Sol's, 68
percent of its diameter,
(Schmitt
et al, 1994; from
Joy
and Sanford, 1926) and 2.6 percent of its
visual luminosity.

Life Around a Flare Star

Many dim, red (M) dwarf stars exhibit unusually violent
flare activity for their size and brightness. These
flare stars are actually common because red dwarfs make
up more than half of all stars in our galaxy. Although
flares do occur on our Sun every so often, the amount
of energy released in a solar flare is small compared
to the total amount of energy that Sol produces.
However, a flare the size of a solar flare occurring on
a red dwarf star (Castor Cab) that is more than ten
thousand times dimmer than our Sun would emit about
as much or more light as the red dwarf does normally.

Flare stars erupt sporadically, with successive flares
spaced anywhere from an hour to a few days apart. A
flare only takes a few minutes to reach peak brightness,
and more than one flare can occur at a time. Moreover,
in addition to bursts of light and radio waves, flares on
dim red dwarfs may emit up to 10,000 times as many X-rays
as a comparably-sized solar flare on our own Sun, and so
flares would be lethal to Earth-type life on planets near
the flare star. Hence, Earth-type life around flare
stars may be unlikely because their planets must be
located very close to dim red dwarfs to be warmed
sufficiently by star light to have liquid water (around
0.23 AU for Castor Cab), which makes flares even more
dangerous around such stars. In any case, the light
emitted by red dwarfs may be too red in color for
Earth-type plant life to perform photosynthesis
efficiently.

Closest Neighbors

The following table includes all star systems known to be
located within 10 light-years (ly), plus more bright stars
within 10 to 20 ly, of Castor.

Constellation Gemini, the Twins, is the northernmost of the
zodiacal constellations and among the brightest. In Greek
mythology, Zeus (the chief of the gods) seduced Leda (the
wife of the King of Sparta, Tyndareos) on her wedding night
by changing himself into a swan. In time, Leda gave birth
to the twin boys immortal Pollux (by Zeus) and mortal
Castor (by Tyndareos), and to a girl named Helena
who became Queen of Sparta and was abducted by Paris to Troy
which led to the Trojan War. The twins, on the other hand,
sailed with Jason in the quest for the Golden Fleece; during
a storm, they helped save their ship ARGO from sinking, and
so the constellation became much valued by sailors.
For more information about the stars and objects in this
constellation and an illustration, go to Christine Kronberg's
Gemini.
For an illustration, see David Haworth's
Gemini.

For more information about stars including spectral and luminosity
class codes, go to ChView's webpage on
The Stars of
the Milky Way.